1. Field of the Invention
This invention relates to a symmetrical proof-mass accelerometer with a self-diagnosis capability and a method of fabricating the same, and more particularly to an accelerometer which is designed to have a symmetrical mass distribution with respect to the plane of a cantilever beam with an offset between upper and lower mass portions, so that cross-axis sensitivity thereof is reduced and self-diagnostic resistor installation is facilitated. This invention can be applied to automobile electronics systems, as well as to consumer electronics and industrial electrical measurement systems, in which measurements of displacement, velocity, vibration, acceleration, angular acceleration and their changes are required.
2. Discussion of the Related Art
Conventional accelerometers with self-diagnosis ability and methods of fabricating the same can be classified into two types. FIG. 1(A) shows an unsymmetric proof-mass accelerometer, in which a self-diagnostic resistor or conductor is installed on a highly-stressed area for detection of the breakage of a cantilever beam.
The other type of the conventional accelerometer is a symmetrical proof-mass accelerometer as shown in FIG. 1(B). In this case, a self-diagnostic resistor or conductor extends out to the level of a mass and support.
These conventional accelerometers and their fabrication methods have problems as follows.
First, the unsymmetric accelerometer as shown in FIG. 1(A) has the problem of relatively high cross-axis sensitivity compared to that of the conventional symmetric accelerometer, because the acceleration in the unwanted direction could cause a defect in the cantilever beam. On the other hand, the conventional symmetric accelerometer as shown in FIG. 1(B) has a difficulty in installing a self-diagnostic resistor or conductor at the ends of the cantilever beam, 9a' and 9b', which are the weakest regions of the accelerometer, since the surface formed by the beam, mass and supporters are not in the same plane.
The common problem in the conventional accelerometers shown, e.g., in FIGS. 1(A) and 1(B) is that the abrupt change in thickness at each end of the cantilever beam generates stress concentration. Therefore, it may cause breakage during the fabrication process or use.
Furthermore, the conventional methods for controlling the thickness of cantilever beams in the conventional accelerometers shown in FIGS. 1(A) and 1(B) includes, such as, time-controlled etching, p-n junction forming and p.sup.+ etch-stop steps. These conventional methods, however, have problems due to process instability, complexity in processing, difficultly in thickness control and limitations in material selection.
The accelerometer according to the present invention overcomes the above and other problems in the conventional methods and accelerometers. The accelerometer of the present invention has two proof-masses having an identical mass with different relative locations upon and beneath the cantilever beam, so that both self-diagnostic element accommodation and cross-axis sensitivity reduction are possible. In addition, the beam thickness control process has been simplified by an etch-thickness difference, and yield and reliability can be enhanced by using a fillet-rounding formation process. The fillet-rounding process forms a curvature at each end of the beam (the weakest region) so as to prevent breakage during or after fabrication.